Geography
First Advisor
Paul R. Hanson
Date of this Version
12-2018
Abstract
The Nebraska Sand Hills is a vast (7500 square kilometer) area of grass-stabilized sand dunes. Larger dunes in the Nebraska Sand Hills formed primarily during the Late Pleistocene, but many underwent widespread reactivation during the Holocene. Recent Optically Stimulated Luminescence (OSL) dating indicates that the last major phase of reactivation in the Sand Hills was during the Medieval Warm Period, approximately 800 years ago. Nevertheless, many questions about the evolution of the dunes remain unanswered, particularly regarding the formation of linear dunes in portions of the Sand Hills.
This study seeks to understand more about the formation of linear dunes and contribute to the current body of knowledge regarding the Nebraska Sand Hills’ geologic past. The linear dune field investigated in this thesis is located in northwestern Brown County, Nebraska along the southern margin of the Niobrara Valley. These dunes range from 300-600 meters in length, and their relief ranges from 6-10 meters. LiDAR imagery acquired in 2012 reveals that the crests of many of the dunes have characteristic “Y”-shaped junctions in plan view, features that have not yet been fully examined in the Nebraska Sand Hills. Additionally, these dunes are not superimposed onto other dune forms, unlike the majority of linear dunes previously studied in the Nebraska Sand Hills. These dunes are interpreted as simple vegetated linear dunes based on their morphology.
Three ground-penetrating radar (GPR) lines, created with 100 MHz frequency antennas penetrated to the bases of the Brown County dunes. The GPR lines depict beds that dip predominately to the south in each dune, unlike previous studies that showed bidirectional dip angles for some linear dunes in the Sand Hills. OSL samples were collected from twelve sediment cores and five hand-auger holes. Twenty-five eolian ages show these dunes stabilized around 2600 years ago and between 950 to 520 years ago, times that correlate to significant periods of drought and dune activity previously recorded in the Nebraska Sand Hills. Several of the deeper (35-40 m) cores contain alluvium that underlies the dunes. On the basis of four OSL ages, the terrace fill underlying the dunes dates to approximately 23,000 to 15,300 years ago.
Advisor: Paul R. Hanson
Comments
A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Arts, Major: Geography, Under the Supervision of Professor Paul R. Hanson. Lincoln, Nebraska: December, 2018.
Copyright (c) 2018 Ashley K. Larsen